1. Field of the invention
The present invention relates to a switching circuit for controlling, e.g., AC power, and also to a relay device employed therein.
2. Description of the Prior Art
A switching circuit is known which is defined by a combination of one or more relay devices and a semiconductor switching element, such as a thyristor circuit or a triac circuit. The prior art switchng circuit is so arranged that the semiconductor switching element is connected directly in series with a load and a power source. To start the power supply, the semiconductor switching element is turned on by a suitable gate signal. Then, during the power supply, the current constantly flows through the semiconductor switching element, thereby undesirably heating the semiconductor. This may result in a breakdown of the semiconductor. To prevent such heating, a bypass circuit is provided parallelly to the semiconductor switching element in such a manner as to close the bypass circuit after the switching element turns on, and to open the same before the switching element turns off. Thus, during the power supply, other than the moments for starting and cutting the power supply, the current flows through the bypass circuit, thereby preventing the switching element from being heated up undersirably.
However, the above switching circuit has the following problems. The first problem is the difficulty in controlling the semiconductor switching element and the bypass circuit in a predetermined timed relationship with each other. For example, if the semiconductor switching element is turned off first and then the bypass circuit is cut, undesirable arc current may be produced in the contacts in the bypass circuit, resulting in the generation of undesirable surge. Also, such an arc current may damage the contact points. The second problem is the breakdown of the semiconductor switching element. Although the bypass circuit is provided to protect the semiconductor switching element, the surge may be applied to the semiconductor switching element, resulting in the breakdown of the same. When this happens, the control of the current flowing through the semiconductor switching element will be lost and, thus, the current constantly flows through the load.